Posted
by
Hemos
on Thursday August 16, 2001 @04:45PM
from the ka-BLAM dept.

msheppard writes "This ariticle at Scientific American describes a new computer model describing the formation of the moon. "It is also strikingly similar to the first proposed impact theory 25 years ago, before computer simulations were available.""

Except the article refers to a consensus reached 25 years ago, but I believe the actual "collision with a Mars-sized body" consensus came from the Kona, Hawaii meeting in 1984 [asi.org]. So that's only 17 years... And basically this model is just an incremental improvement (will a big increment - 20,000 body simulation instead of 3,000) over previous simulations of the process. Still interesting though!

While it isn't likely that present day Earth will encounter another impactor such as may have formed the moon, legitimization of the theory behind this model goes a long way to giving planetary astronomers a better understanding of how planetary systems are formed. If they can't explain how WE got here, then its really difficult to conclude one way or another that similar systems are or are not out there.

I'm not against gathering evidence, but a conclusion is the worst possible thing that can happen in science.

If one concludes that something is true, then the subject looses interest. People loose interest in the subject. Lines of research loose funding. The theory isn't challenged. Potential for advancement is lost.

Another theory (possibly posed by less sober astronomers) is that the moon is actually an orbiting landfill from the civilization which predated our own, by hundreds of millions of years. The theory goes that there have actually been 4 or 5 civilizations on earth, each wiped clean by a cataclysm (i.e. buried under the weight of styrofoam fast-food containers) and the earth would enter a phase of lower life forms which would tromp around, florish then die, providing fossil fuels for the next civilization.

On a separate, but slightly related angle, there was a paper released a couple months back (see CNN Story) [cnn.com] that came to the conclusion that something very weird happened in the Solar system about 65 million years ago. Studies of ocean sediment patterns reveal that the earth has been going through a 400,000 year climate cycle that is directly related to planetary distance. The problem is that these patterns change at about 65 million years ago. This is obviously related to the asteriod thast knocked of the dinosaurs.

Fringe groups have been looking at this and speculated that this is when the asteroids were formed, and when mars got its weird pattern of craters that cover only half the planet. You can download a nicely done 60 page document of this sort of thing (PDF [enterprisemission.org] - HTML [enterprisemission.com]). Unfortunately, the authors like to occasionally bring in things that are not relevant, so it sort of ruins the flavor, but it is not bad, and interesting reading, even if you do not take it seriously.

Which of course goes brings up all kinds of interesting speculations on just how common are planetary collisions in the first place.

Basically the article confirms what we all already knew (most of us learning it from the Carl Sagan COSMOS series). They have a model which proves that a solid body mass "escaped from the inner gases of Jupiter" and eventually slammed into the Earth. "The impact caused an extensive spray of material out into space, which gradually coalesced into the Moon."

The only time I recall hearing Sagan mentioning anything like this on Cosmos was when he was debunking the theories of Immanuel Velikovsky [varchive.org]. That particular presentation did not culminate in the expelled body colliding with Earth, but rather settled down into orbit around the Sun to become the planet Venus. You must be confusing two different segments.

I don't know where you got that. It was in neither the SciAm article nor the Cosmos series. It sounds like you have confused Carl Sagan with the crackpot Immanuel Velikovsky, which no doubt has the former turning in his grave.

And I might have done, except so many people take this kind of thing seriously. Yours was not the most insane bit of local astronomy I've heard, and usually it's stated with all earnestness. Check this out [sitchin.com], for instance.

Is this really the case? Last time I sat through astronomy in college they still seemed to make a lot of the "concurrent development" theory. Has that been shown wrong recently?

The problem is (or so I believe, never having taken an astronomy course in my life) that the Earth has a large amount of iron in its core - whereas the moon has almost no iron. If the Earth and moon coalesced out of the same matter at the same time, you'd expect them to have a similar composition.

The easiest way to explain the difference was to claim that the Earth was hit by another object which sloughed off part of the surface, leaving the core largely unchanged. The problem has been getting the timing and mass right: this simulation shows that it had to be about a Mars-sized object at a specific time - any bigger or smaller, or at a different time, and you need multiple collisions to obtain the Earth and moon as we see them today.

-dair (like I say, not an astronomer - just going by what I read in the paper this morning)

I'm no astrophysics guru, but this simulation seems to merely confirm the possible a theory rather than put forward a valid creation model as a contender for cannon. Wouldn't the same sort of thing have to be done for every other possible creation scenario before we could get anything approaching conclusive confirmation?

Showing that something works is inconclusive . . . showing that everything else doesn't work is better.

Example: Two is the only even prime number.
Proof: Two can be divided by one and two. Viola!

Exactly . . .
I chose the proof to show that in order to demonstrate something as a unique property (say, the way the moon was formed), you need to not only show that it has the property but also show that it is the only one that has the property (no other formation model is valid).

This is a valid point, but in the defence of others I don't think that most people are accepting this as the obvious conclusive proof. It is at best a simulation, and is therefore a potentially instructive tool, like molecular modelling. I think that the indistinguishability of the state of matter is interesting, but I don't see how replacing the ejection of solid debris with gaseous or liquid will significantly alter anything, given the mass of the proposed participants. But I am certain that more than one TV station will run a story about the absolute proof of the origin of the moon being discovered. I am also betting that more than one of these newscasts will show their dim-witted viewers that all such astrophysical phenomena can be described by filming a rather rotund and inebriated individual causing billiard balls to strike one another in a seedy establishment known colloquially as a "pool hall".

You make the mistake of confusing mathematics with sciences. The two are related, but not the same. Math allows for proof, science does not. Science only permits us to say that we think that one theory is more likely than another.

As to your question, there are 4 ways that have been suggested to make a moon:
1) Form it in place. This is probably true of Juptier's Galilean satellites.
2) Capture it. This is probably true of most of the irregular satellites and possibly Mars's moons.
3) Fission it off of the planet via over-spinning. Suggested first by Darwin's son, by way of interest.
4) Giant impact (or "Big Wack"). Probably true of Earth's Moon and Pluto's Charon.

So why do we opt for number 4? Capture is hard. It is feasible for big gasy planets and small moons because they can get rid of the extra energy by gas drag (we think). There isn't enough angualr momentum in the Earth/Moon system for fission (there might be in Pluto/Charon). The Moon is too big to have formed in place in current formation models and, in any event, ought to be made of the same stuff as Earth. It is composed, almost straight through, of material like Earth's mantel, but not the core.
All in all, the giant impact model seems to beat out the competition soundly. So why does Robin keep doing these simulation? Well, the details are interesting and important for understanding the details of the result. Additionally, we would like see this attention turned to Pluto/Charon now. No one has done that simlulation yet, they've just extrapolated off of the Earth/Moon work. This is risky because Pluto is made of ice, not rock, and behaves differently. Robin has been, when I last spoke to her in May, working on getting the ice behavior right in her models. Hopefully, we'll see about Pluto/Charon soon.

In a sense you're right, but you need more context. It sometimes happens in science that an observed phenomenon (in this case the moon) proves hard to explain in a satisfactory way (whatever that means) at all. In such a case, it becomes interesting whenever anyone comes up with any plausible explanation that seems to work. This is an example of this. It is hard to come up with any explanation which gives us such a large moon with no iron core and so on.

Another example is string theory. It is observable that quantum mechanics works extremely well at low energies and that general relativity works extremely well at large scales. Unfortunately, they don't work well together at high energies and small scales. ANY theory which manages to match them up and looks like it might have some predictive value eventually is interesting.

used a highly detailed computer modeling system--one that divided the Earth and its impactor into more than 20,000 tiny particles

if you divided the surface of the United States into 20,000 longitudinal (sp?, or even a word?) slices of equal width, these "tiny particles" would be in excess of 500ft wide. That's much smaller than if you did it with the whole earth (plus whatever hit it). I dunno, maybe that's all the resolution needed to predict the outcome of celestial events. I'd tend to doubt it. This sounds like a *real* rough and dirty approximation, hardly worth the money it cost to build.

If you want to know if your computer model is using a fine enough grid (enough particles) there's a simple way to be sure. Run the simulation with 5000 particles, then 10000, then 20000, etc. Keep doing it until the answer doesn't change much, and you can be pretty sure you have enough particles. Of course, I don't know that they did something like this, but some sort of grid resolution study is pretty standard in any advanced computer simulation. I'd be extremely surprised if they couldn't justify using 20000 particles.

Not really. The collision must have been catastrophic for the other body. If it was Mars-sized, then it would have been a little over 1/10 the mass of the Earth. That's big enough to throw a considerable amount of the Earth's material into orbit, but also small enough that it would have been completely pulverized itself. Whatever pieces did not coalesce along with the other material into the Moon or become absorbed by the Earth would have simply contributed to the Solar System's collection of asteroids.

It amazes me that so many allegedly "educated" people have fallen so quickly and so hard for a fraudulent fabrication of such laughable proportions. The very idea that a gigantic ball of rock happens to orbit our planet, showing itself in neat, four-week cycles -- with the same side facing us all the time -- is ludicrous. Furthermore, it is an insult to common sense and a damnable affront to intellectual honesty and integrity. That people actually believe it is evidence that the liberals have wrested the last vestiges of control of our public school system from decent, God-fearing Americans (as if any further evidence was needed! Daddy's Roommate? God Almighty!)

Documentaries such as Enemy of the State have accurately portrayed the elaborate, byzantine network of surveillance satellites that the liberals have sent into space to spy on law-abiding Americans. Equipped with technology developed by Handgun Control, Inc., these satellites have the ability to detect firearms from hundreds of kilometers up. That's right, neighbors.. the next time you're out in the backyard exercising your Second Amendment rights, the liberals will see it! These satellites are sensitive enough to tell the difference between a Colt.45 and a.38 Special! And when they detect you with a firearm, their computers cross-reference the address to figure out your name, and then an enormous database housed at Berkeley is updated with information about you.

Of course, this all works fine during the day, but what about at night? Even the liberals can't control the rotation of the Earth to prevent nightfall from setting in (only Joshua was able to ask for that particular favor!) That's where the "moon" comes in. Powered by nuclear reactors, the "moon" is nothing more than an enormous balloon, emitting trillions of candlepower of gun-revealing light. Piloted by key members of the liberal community, the "moon" is strategically moved across the country, pointing out those who dare to make use of their God-given rights at night!

Yes, I know this probably sounds paranoid and preposterous, but consider this. Despite what the revisionist historians tell you, there is no mention of the "moon" anywhere in literature or historical documents -- anywhere -- before 1950. That is when it was initially launched. When President Josef Kennedy, at the State of the Union address, proclaimed "We choose to go to the moon", he may as well have said "We choose to go to the weather balloon." The subsequent faking of a "moon" landing on national TV was the first step in a long history of the erosion of our constitutional rights by leftists in this country. No longer can we hide from our government when the sun goes down.

Of course, this all works fine during the day, but what about at night? Even the liberals can't control the rotation of the Earth to prevent nightfall from setting in (only Joshua was able to ask for that particular favor!)

(emphasis mine)

But then our friend goes on to claim:

There is no mention of the "moon" anywhere in literature or historical documents -- anywhere -- before 1950.

Nice try, but you just referenced Joshua 10:12 in the Old Testament (written WAY before 1950, something that EVERYONE will agree on...):

12 Then Joshua spoke to the LORD in the day when the LORD delivered up the Amorites before the sons of Israel, and he said in the sight of Israel, "O sun, stand still at Gibeon, And O moon in the valley of Aijalon."

>> There is no mention of the "moon" anywhere in
>> literature or historical documents -- anywhere --
>> before 1950.
>
> Nice try, but you just referenced Joshua 10:12 in
> the Old Testament (written WAY before 1950,
> something that EVERYONE will agree on...):

Hey, pal! I'm sorry to burst your bubble, but there was no mention of the BIBLE in any records at all before 1951, when Ike, in a campaign speech, mentioned those godless, Bible-stomping commies in China...

The movie you reference was clearly about the Conservative government, as was "Conspiracy Theory."

Its "Primary Color's", "Wag the Dog" and "Dave" that are about the liberal government. Yep just happy go lucky, comical *harmless* shmoes you'd rather have over for dinner anyway. We^H^HThey have no sophisticated tracking equipment or anything else to be afraid of. Just love and concern for every American.

hey guys, i'm actually working at southwest research - no, not working on this project, but rather with their video department. But we're setting up the video of the simulation to go off the web. it's not much (a 400k 16sec clip) but we've got it available to view in.wmv format here:

http://129.162.26.201/photolab/moon05.wmv

actually, the idiots who were in charge of the press release on the web stuck some huge 720x480 avi on the web earlier today and made our netowrk grind to a halt. They've since removed that file. (serves them right i guess for not checking with us video guys first about it)
I can assure you that this one is much smaller.

and since i probably shouldn't be posting this(yet), i've decided to be an AC

oops i missed the "post anonymously" button and hit the submit one instead. oh well. like i'm really _that_ concerned about my job. (i'll probably put in notice in the next week or so anyway - too much burocracy/mis/non-communication. and that's in the communications department!) sorry to rant. it's been chapping my hide for a while now.

I confess to ignorance, but I understood the specific gravity of the moon was so different to Earth's, that it could not have been formed from material that originally came from here. Unfortunately, it couldn't be re-ejected material from the impactor either... because that would be half-way to Earth's core and not in a hurry to pop back into space.

The composition of the body doesn't matter, at least, not in the way you're talking about. What matters is the total mass -- As you'll find in any physics textbook, gravity between two objects is determined by the formula GM1M2/R-Squared, or in the case of one object being significantly larger than another, i.e. a person to the Earth/Moon, or the Earth to the Sun, simply GM/R^2, where M is the mass of the big object.

When the object collided with the earth, both "shattered," so to speak. But much of the mass of Earth stayed where it was, while some of it was shot off. The Earth, or, what was left of it, had enough gravity to keep some of the pieces that were trying to break free in place, and the angular momentum (which was, of course, conserved), put the object into orbit. Which is why, of course, the moon doesn't just come crashing down. Or why we don't just get sucked straight into the Sun.

yeah - understand that much. Read somewhere that the moons composition was very unlike earth's though - ie this material isn't a part of the earth, and couldn't be the remains of anything that hit the earth either.

...that guy could buy Bill Gates, and still have enough left over to purchase quite a few elected officials.

Does he really think that the beam from the average keychain laser pointer could go 238,000+ miles? [eb.com]

What will his next project be? Hooking his laser pointer up to a car battery, putting a telescopic sight on it, and telling us he's augmented his local Neighborhood Watch with its very own homespun missile defense system?

The government should monitor this "project," round up everyone who participates, and unleash them in Iraq or something.

If people are wondering if this will work, try this: go to one end of a corridor and try to keep the light fixed on something at the other end. It's almost impossible to keep it stable at 50 meters or so - people would be luck to hit the moon at all.

I could probably look this up in a 6th grade science book, but why the hell are the planets generally round if such large objects have smashed into them. I wouldn't think there'd be any friction between a planet's surface and space. And if it is from erosion, then how long would such a process take?

Just from general observations, when two things smash into each other they generally don't come off as 2 round pieces.

Not a stupid question. The answer is that the gravitational forces exceed the ability of the planet's materials to resist them. Thus the planet's shape assumes that which minimises the gravitationally induced stress, i.e. the sphere. Where the internal gravitational forces are weak, for example within your car or the asteriod Eros, this doesn't happen.

All you need is enough size in the object. A molten interior helps, but I don't think is essential. If you think of the planet as a loose collection of many objects rather than a single solid object, gravity tends to act just as with a liquid or gas, bringing everything to an 'equipotential surface', i.e. a sphere. The spinning of the Earth actually distorts its shape slightly from a sphere. But it's basically a matter of size (and density). As you get down to smaller bodies, in particular the asteroids, they get less and less spherical - you've probably seen pictures of Eros from the NEAR mission: basically a dumb-bell shape, not a sphere at all.

but why the hell are the planets generally round if such large objects have smashed into them...when two things smash into each other they generally don't come off as 2 round pieces

They do if they're liquid. Early on, the inner planets were big balls of molten rock. Irregular objects - like many of the smaller asteroids - are smashed-up bits of larger things that were round but cooled down before they got smashed.

There are a few mysteries that have been solved at least in theory... Approximately 4,000,000,000 million years ago, Venus was hit by a cataclysmic event that reduced it's surface to molten and caused it to recycle it's entire crust, whereas the Earth, when impacted by an unknown planetoid, 400,000,000 years ago, had it's crust reduced to a semi molten state, sufficient to release the lunar material... In either case though, rotation and gravity resulted in them having a spherical shape...

In the case of Mars, topographical evidence shows it may have had a large body impact it's surface, or even one sufficient to cleave away a good chunk of it's surface, such that it's actually slightly lopsided by the amount of missing landmass, to the point where it's obvious it could have happened far more recently...